The method and apparatus of the present invention are applicable to the hot-dip coating of a ferrous base metal strip with zinc, zinc alloys, aluminum, aluminum alloys, terne and lead. While not intended to be so limited, for purposes of an exemplary showing, the method and apparatus of the present invention will be described as applied to galvanizing and aluminum coating.
The utilization of an inert or non-oxidizing atmosphere in association with the finishing step of a hot-dip coating process is not new in and of itself. For example, U.S. Pat. Nos. 4,107,357 and 4,114,563 and German Patent No. 2,656,524 are exemplary of patents teaching methods for coating one side only of a ferrous base metal strip. In the practice of these processes, the coated strip (after contact with the coating bath) is maintained in a protective, non-oxidizing atmosphere and is jet finished with nitrogen or non-oxidizing gas. The primary purpose of these steps is to prevent oxidation of that side of the ferrous base metal strip not coated.
U.S. Pat. Nos. 3,505,042 and 3,505,043 teach a method of hot-dip coating a ferrous base metal strip with a zinc-magnesium-aluminum coating. The strip is rapidly cooled by a non-oxidizing or reducing atmosphere until the coating solidifies to prevent or minimize oxidation of the magnesium in the coating.
U.S. Pat. No. 4,330,574 teaches a method of finishing a two-side coated ferrous base metal strip in a conventional continuous hot-dip coating line. The strip, exiting the coating metal bath, is maintained in an enclosure and subject to jet finishing within the enclosure. The jet finishing is accomplished with an inert or non-oxidizing gas and the jet finishing gas and the atmosphere within the enclosure are maintained at an oxygen level below about 1,000 ppm. By virtue of this, a number of finishing problems encountered with conventional jet finishing methods are markedly reduced or eliminated. The most significant aspect of this reference is the discovery that all coating control problems at the strip edges, generally encountered in jet finishing, are completely eliminated.
U.S. Pat. No. 2,992,941 teaches the provision of nozzles to either side of the strip in combination with finishing rolls. A blast of gas, such as air, is directed against the meniscus formed between the strip and the finishing rolls on both sides of the strip, the force of such blast being controlled to provide a back pressure to the pumping and dragging actions of the rolls and strip so that by increasing the force of the blast, these actions are impeded and the meniscus on each side of the strip is reduced.
At the present time, most high speed, conventional, hot-dip coating lines utilize jet finishing, rather than finishing rolls, since finishing rolls tend to limit permissible strip speeds. Nevertheless, finishing rolls are in common use in conventional hot-dip coating lines having a strip speed ranging from about 20 to about 150 feet per minute. While not necessarily so limited, many such lines today are used to coat heavier gauge ferrous base metal strips having a gauge or thickness of from about 0.060 inches to about 0.180 inches.
The present invention is based upon the discovery that a number of advantages are obtained in a conventional hot-dip metallic coating line utilizing traditional finishing rolls, when an enclosure is located about the coated ferrous base metal strip as it exits the finishing rolls. Within the enclosure, an inert or non-oxidizing atmosphere is provided to shroud the meniscus areas created by the finishing rolls. These advantages comprise an appearance improvement in the coating from elimination of oxide-related defects, a consistently uniform coating distribution and surface, reduced equipment wear, a reduced requirement for operator attention, an increased operating capacity by permitting higher line speeds without exceeding maximum coating weight specifications, and an improved response to subsequent surface treatment such as a spangle minimization treatment. These advantages will be further described and specified hereinafter.